.box {
    width: 80%;
    min-height: 500px;
    background: skyblue;
    margin: 0 auto;
}

.zzc {
    position: fixed;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    background: rgba(0, 0, 0, 0.3);
    z-index: 0;
    display: none;
}

.mtk {
    width: 450px;
    height: 350px;
    background: #fff;
    position: absolute;
    top: 0;
    left: 0;
    bottom: 0;
    right: 0;
    margin: auto;
    z-index: 1;
    display: none;
    padding: 50px;
    box-sizing: border-box;
}